|
Faunsdale, AL | When you're using mostly 3phase with a little single phase for a few lights, operating coils on starters, maybe an office AC or two etc, go with 240. It is usually supplied with an open delta transformer bank and the phase balance will depend on getting the transformers sized correctly. I believe most of the problems which are historically associated with these services are because they were originally set up for mostly single phase with a single or very limited 3 phase load. Phase voltage balance was not very good when the transformer bank was built to the minimum size.
120/208-Y 3 phase is good for commercial buildings with lots of 120 volt lighting and receptacle circuits to supply because these circuits can be applied to all 3 phases and keep the load balanced as opposed to the 120/240 3 phase where these loads can only be applied to 2 out of the 3 phases and cause voltage balance problems.
Running a 3 phase motor, if there is a voltage difference between any of the 3 phases, there is a resulting amperage imbalance about 3 times as great. So anything more than a small voltage difference results in derating of the motors. I know on our farm, a motor which is fully/over loaded has a shortened life compared to one which has a little room.
Motors sold in North America typically have a service factor rating on the tag above their rated full load amps. The motor should handle the load as long as the service factor amp rating is not exceeded, but life of the motor that operates full time in the service factor area is shortened somewhat. Operating on imbalanced voltages makes the motor operate hotter and decreases the safe amperage rating, I would recommend not operating in the service factor when on 120/240 3 phase because the voltages are not typically balanced as well as the 120/208Y services.
You can still have imbalance on the Y services from line voltage imbalance on the utility high voltage lines as well. When everyone comes home from work and turns on the AC, the washer and dryer, and the kitchen stove, sometimes the load isn't too well balanced and incoming voltage will go change more on one phase than the others. Usually have to put on a data logger to document this, but it's worth considering if you have a problem with motors lasting.
Of course, phase loss protection is a very good investment for any 3 phase motors. (loss of a phase is the worst voltage imbalance possible, so it results in rapid overheating of all but the most lightly loaded motors) Phase loss/reversal protection is available in some starters, or can be an add-on to perhaps shut down a whole system at once if a phase is lost. Modern motor starters with a properly adjusted overload relay offer some phase loss protection, but will not always save a motor that is overloaded or getting imbalanced voltage. Best to go with true phase loss/reversal detection in addition to the overload unless it is incorporated into the design like some are.
Edited by ccjersey 7/15/2012 11:55
| |
|
Which electric power should I use?
